U.S. patent number 6,826,906 [Application Number 09/816,912] was granted by the patent office on 2004-12-07 for exhaust system for enhanced reduction of nitrogen oxides and particulates from diesel engines.
This patent grant is currently assigned to Engelhard Corporation. Invention is credited to Karl R. Grimston, Ramesh M. Kakwani, Joseph A. Patchett, Kenneth C. Voss.
United States Patent |
6,826,906 |
Kakwani , et al. |
December 7, 2004 |
Exhaust system for enhanced reduction of nitrogen oxides and
particulates from diesel engines
Abstract
A diesel engine aftertreatment exhaust system uses catalyzed
soot filters for particulate matter reduction and urea SCR
catalysts for NOx reduction on diesel engines in a combined system
to lower particulate matter and NOx at the same time. With this
integral emission control system, diesel engines are able to meet
ultra low emission standards.
Inventors: |
Kakwani; Ramesh M. (Whitehouse
Station, NJ), Voss; Kenneth C. (Somerville, NJ),
Patchett; Joseph A. (Basking Ridge, NJ), Grimston; Karl
R. (Upper Strensham, GB) |
Assignee: |
Engelhard Corporation (Iselin,
NJ)
|
Family
ID: |
26919632 |
Appl.
No.: |
09/816,912 |
Filed: |
March 23, 2001 |
Current U.S.
Class: |
60/303; 60/286;
60/297; 60/295; 60/292 |
Current CPC
Class: |
F01N
3/2066 (20130101); F01N 13/0097 (20140603); F01N
13/009 (20140601); B01D 53/9409 (20130101); F01N
3/0231 (20130101); F01N 3/035 (20130101); F01N
13/011 (20140603); Y02A 50/2325 (20180101); Y02C
20/10 (20130101); F01N 2510/06 (20130101); B01D
2255/20738 (20130101); F01N 2610/02 (20130101); Y02T
10/12 (20130101); Y02T 10/24 (20130101); B01D
2255/50 (20130101); B01D 2258/012 (20130101); F01N
2510/063 (20130101); Y02A 50/20 (20180101); B01D
2251/2062 (20130101); F01N 2610/03 (20130101) |
Current International
Class: |
F01N
3/20 (20060101); F01N 3/035 (20060101); F01N
3/023 (20060101); F01N 7/00 (20060101); F01N
7/02 (20060101); F01N 003/10 () |
Field of
Search: |
;60/274,286,301,303,295,297,311 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 498 598 |
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Aug 1992 |
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EP |
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0 806 553 |
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Nov 1997 |
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EP |
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1 054 722 |
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Dec 2001 |
|
EP |
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WO 99/39809 |
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Aug 1999 |
|
WO |
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WO 00/21647 |
|
Apr 2000 |
|
WO |
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WO 00/29726 |
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May 2000 |
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WO |
|
Other References
Publication entitled "Nitrogen Oxides Control Technology Fact
Book", pp. 84-105, dated 1992, by Sloss, Hjalmarsson, Soud,
Campbell, Stone, Shareef, Maibodi, Livengood, Markussen, Noyes Data
Corporation, Park Ridge, NJ, USA. .
SAE paper No. 930363, entitled "Off-Highway Exhaust Gas
After-Treatment: Combining Urea-SCR, Oxidation Catalysis and
Traps", by Hug, Mayer and Hartenstein, dated Mar. 1-5, 1993,
International Congress and Exposition, Detroit, Michigan, USA.
.
Brochure entitled "Exhaust Gas Purification Systems", by Hug
Engineering, dated 1996, pp. 2-7. .
Brochure entitled "STARU--Stationary gas purification systems", by
Hug Engineering, dated 1996..
|
Primary Examiner: Tran; Binh Q.
Attorney, Agent or Firm: Negin; Richard A.
Parent Case Text
CROSS REFERENCE TO PATENT APPLICATION UNDER 35 USC .sctn.119
This application claims the benefit of U.S. Provisional Application
No. 60/225,478, filed Aug. 15, 2000, entitled "EXHAUST SYSTEM FOR
ENHANCED REDUCTION OF NITROGEN OXIDES AND PARTICULATES FROM DIESEL
ENGINES".
Claims
Having thus defined the invention, it is claimed:
1. An emission purification system for treating exhaust gases
produced by a vehicle powered by a diesel engine comprising: a) a
catalyzed soot filter adjacent and in direct fluid communication
with said engine without intervening catalysts therebetween, said
soot filter of the wall-flow type having gas permeable walls formed
into a plurality of axially extending channels, each channel having
one end plugged with any pair of adjacent channels plugged at
opposite ends thereof, said exhaust gases passing through said
channel walls as said gases travel from an entrance to an exit of
said soot filter; b) a valve downstream of said soot filter's exit
in fluid communication with a nitrogen reductant and with said
exhaust gases after exiting said soot filter; c) means for
regulating said valve to control the quantity of said nitrogen
reductant admitted to said exhaust gases; and, d) a nitrogen
reductant SCR catalyst downstream of said valve and said soot
filter in direct fluid communication with said soot filter, said
SCR catalyst having a set temperature at which said SCR catalyst
becomes catalytically active for a set space velocity if said
exhaust gases pass through said SCR catalyst with a set quantity of
reductant immediately upon exit from said engine that is higher
than the temperature at which said SCR catalyst becomes
catalytically active when said exhaust gases pass through said SCR
catalyst at said set space velocity with said set quantity of
reductant after passing through said soot filter.
2. The system of claim 1 wherein said soot filter has a catalyzed
surface containing at least 25 g/ft.sup.3 of a precious metal
coating.
3. The system of claim 1 wherein said SCR catalyst has a catalyst
composition of zeolite, a promoter selected from the group
consisting of iron and copper and a refractory binder.
4. The system of claim 3 wherein said nitrogen reductant is ammonia
and said quantity of said reductant metered to said exhaust gases
does not exceed a normalized stoichiometric ratio of 1.5.
5. The system of claim 4 wherein said catalytically active
temperature of said SCR catalyst downstream from said soot filter
is less than about 200.degree. C.
6. A method for treating exhaust gas emissions produced by a
vehicle powered by a diesel engine including light duty diesel
engines, said exhaust gases including nitrogen oxides, NOx, with
nitric oxide (NO) comprising at least 50% of the composition of
said NOx, and soot containing a VOF, said method comprising the
steps of: a) providing a catalyzed soot filter downstream of said
engine, said soot filter comprising gas porous walls catalyzed on
both sides thereof formed into axially extending channels, each
channel having a plug at one end and open at its opposite end with
any pair of adjacent channels having plugs at opposite channel
ends; b) flowing said exhaust gas produced by said engine without
any catalyzing device altering the composition of said exhaust gas
into channels having open ends confronting said engine which define
open end channels, oxidizing said NO through contact with said
catalyzed wall surfaces of said open ended channels to produce
NO.sub.2 and reacting said NO.sub.2 with said soot in said open
ended channels and is reduced from SCR catalyst said NO.sub.2 to
said NO; c) flowing said NO through said walls into channels having
plug ends confronting said engine which define closed end channels,
and oxidizing said NO by contact with said catalyzed wall surfaces
on said closed end channels to produce NO.sub.2, said exhaust gases
having a higher concentration of NO.sub.2 exiting said soot filter
than entering said soot filter; d) injecting a set amount of a
nitrogen reductant into said exhaust stream downstream of said soot
filter; e) providing a SCR catalyst on a monolith; and, f) passing
said gases into which said reductant has been injected over and in
contact with said SCR catalyst whereby said NOx is reduced.
7. The method of claim 6 wherein said soot filter has a catalyzed
surface containing at least 25 g/ft.sup.3 of a platinum metal
group.
8. The method of claim 7 wherein said SCR catalyst has a catalyst
composition of zeolite, a promoter selected from the group
consisting of iron and copper and a refractory binder.
9. The method of claim 8 wherein said nitrogen reductant is ammonia
and said quantity of said reductant metered to said exhaust gases
does not exceed a normalized stoichiometric ratio of 1.5.
Description
BACKGROUND
A) Field of Invention
This invention relates generally to a diesel exhaust aftertreatment
system and more particularly to a diesel exhaust treatment system
that simultaneously provides for a high level reduction of nitrogen
oxides (NOx) and particulate emissions under lean engine operating
conditions.
B) Incorporation by Reference
The following United States patents are incorporated by reference
herein and made a part hereof. Specifically, the compositions of
the catalysts disclosed in the patents below and how the
compositions are made and/or applied to the disclosed filter or SCR
catalysts are incorporated herein so that such material need not be
repeated or explained again in detail in the Detailed Description
of this invention. U.S. Pat. No. 4,833,113 to Imanari et al.,
issued May 23, 1989, entitled "Denitration Catalyst for Reducing
Nitrogen Oxides in Exhaust Gas"; U.S. Pat. No. 4,961,917 to Byrne,
issued Oct. 9, 1990, entitled "Method for Reduction of Nitrogen
Oxides with Ammonia Using Promoted Zeolite Catalysts"; U.S. Pat.
No. 5,100,632 to Dettling et al., issued Mar. 31, 1992, entitled
"Catalyzed Diesel Exhaust Particulate Filter"; and, U.S. Pat. No.
5,804,155 to Farrauto et al., issued Sep. 8, 1998, entitled "Basic
Zeolites as Hydrocarbon Traps for Diesel Oxidation Catalysts".
While the catalysts disclosed in the patents incorporated by
reference herein may be used in the present invention, they do not,
per se, or, in and of themselves, form the present invention.
C) Prior Art
Compression ignition diesel engines have great utility and
advantage as vehicle power plants because of their inherent high
thermal efficiency (i.e. good fuel economy) and high torque at low
speed. Diesel engines run at a high A/F (air to fuel) ratio under
very fuel lean conditions. Because of this they have very low
emissions of gas phase hydrocarbons and carbon monoxide. However,
diesel exhaust is characterized by relatively high emissions of
nitrogen oxides (NOx) and particulates. The particulate missions,
which are measured as condensed material at 52.degree. C., are
multi phase being comprised of solid (insoluble) carbon soot
particles, liquid hydrocarbons in the form of lube oil and unburned
fuel, the so called soluble organic fraction (SOF), and the so
called "sulfate" in the form of SO.sub.3 +H.sub.2 O=H.sub.2
SO.sub.4.
Both NOx and particulates have been difficult diesel exhaust
components to convert and future emissions standards have been
recently adopted in the US and Europe for both heavy duty and light
duty diesel powered vehicle which are expected to require reduction
of both of these emissions by at least 50% and quite likely by
70-90%.
One commercial aftertreatment technology which has proven very
successful for reduction of NOx under lean exhaust conditions for
stationary sources is Selective Catalytic Reduction (SCR). In this
process NOx is reduced to N.sub.2 with NH.sub.3 over a catalyst
(e.g. zeolite, V/Ti). This technology is capable of NOx reduction
in excess of 90% and thus it is one of the best candidates for
meeting the aggressive NOx reduction goals. SCR is currently under
development for mobile source, vehicle applications using urea
(e.g. aqueous solution) as the source of NH.sub.3. SCR is very
efficient for NOx reduction as long as the exhaust temperature is
within the active temperature range of the catalyst (e.g.
>300.degree. C.). Unfortunately diesel exhaust temperatures are
many times considerably lower than that required for good catalyst
efficiency (i.e., below "light-off"). This is especially true for
light duty (LD) diesel applications such as diesel autos which
operate at light load for the most part, resulting in very low
exhaust temperatures (150-250.degree. C.). Even diesel trucks
operate under conditions which result in exhaust temperatures below
the optimum temperatures for SCR catalysts. Unfortunately, one of
the best, most stable, SCR catalysts, which is of the zeolite type
(e.g. The assignee, Engelhard Corporation's ZNX catalyst, a Fe-Beta
Zeolite), also has the highest optimum operating temperature. As a
result its effectiveness is greatly diminished at diesel exhaust
temperatures of interest.
One key aftertreatment technology under development or very high
level particulate reduction is the diesel particulate filter. There
are many known filter structures that can be used to remove
particulates from diesel exhaust, including honeycomb wall-flow
filters, wound or packed fiber filters, open-cell foams, sintered
metal filters, etc. However, ceramic wall-flow filters have
received the most attention. These filters are capable of removing
over 90% of the particulate material from diesel exhaust and thus
can meet this emissions reduction goal. The filter is a physical
structure for removing particles from exhaust and the accumulating
particles will increase the back pressure from the filter on the
engine. Thus the accumulating particles (soot+hydrocarbons) have to
be continuously or periodically burned out of the filter to
maintain an acceptable backpressure level. Unfortunately, the
carbon soot particles require temperatures in excess of
500-550.degree. C. to be combusted under oxygen rich (lean) exhaust
conditions. This is higher than typical diesel exhaust
temperatures. A means must be provided to lower the soot burning
temperature in order to provide for "passive" regeneration of the
filter. One good way to accomplish this is to provide a suitably
formulated catalyst which is applied to the filter. The presence of
the catalyst has been found to provide soot combustion and thereby
regeneration of the filter at temperatures accessible within the
diesel engine's exhaust under realistic duty cycles. In his way a
Catalyzed Soot Filter (CSF) or Catalyzed Diesel Particulate Filter
(CDPF) can be an effective way to provide for >90% particulate
reduction along with passive burn-out of the accumulating soot and
thereby filter regeneration.
In stationary applications, a number of arrangements routinely use
filters upstream of an SCR catalyst with an ammonia reductant
injected between filter and SCR catalyst. Several arrangements are
disclosed in Nitrogen Oxides Control Technology Fact Book, 1992,
Noyes Data Corporation, pages 84-105. However, all the temperatures
for SCR are high and the filters, discussed generally, are of the
dust particulate type such as electrostatic precipitators.
Hug Engineering AG has developed a gas purification stationary
system described in SAE paper 930363, "Off-Highway Exhaust Gas
After-Treatment Combining Urea-SCR, Oxidation Catalysis and Traps".
In this system, NH.sub.3 is injected upstream of catalyst beds
containing an SCR followed by an oxidation catalyst. In a later Hug
brochure (1996) a soot filter bed (optional) is provided in a
casing adjacent to and upstream of a SCR reactor adjacent to and
upstream of an oxidation catalyst and the urea injected into the
waste gases passing through, in sequence, the filter, SCR and
oxidation catalyst. The soot filter is described as a fibrous
bundle which filters fine soot particles from the exhaust stream
that have a carcinogenic effect. The Hug system disclosed has been
applied to a ferry and other large diesel engine applications
operating for the most part at steady speeds and higher
temperatures than the vehicular applications of the present
invention. A Hug brochure for stationary gas purification systems
describes Hug's "Staru" system in which the soot filter is split
from the SCR and oxidation catalysts with NH.sub.3 injected
therebetween. The soot filter described as fibrous to continue the
function of trapping fine soot particles but is catalytically
coated to regenerate late or burn off the soot at 450.degree. C. In
general, the Hug systems have shown the ability to reduce NOx
exhaust emissions from large diesel engines operating generally
steady state at higher temperatures than light duty diesel engines
by injecting NH.sub.3 upstream of SCR-oxidation catalysts and using
a downstream fibrous, regeneration filter to trap fine soot
particles.
The patent literature discloses U.S. Pat. No. 5,746,989 to Murachi
et al. issued May 5, 1998 and PCT application PCT/GB99/03281
(published Apr. 20, 2000 as WO 00/21647) which use NOx absorbers
that are periodically regenerated. Downstream of the NOx absorber
is an oxidation catalyst and between absorber and oxidation
catalyst is a particulate filter. In the '989 patent, the absorber
is regenerated by varying the A/F ratio and in the PCT application,
NOx reactant is injected upstream of the absorber.
U.S. Pat. No. 4,912,776 to Alcorn issued Mar. 27, 1990 discloses an
oxidation catalyst, an SCR catalyst downstream and adjacent to the
oxidation catalyst, and a reductant source introduced to the
exhaust between the oxidation catalyst and the SCR catalyst.
Consistent with at least one of the theories of the present
invention, the Alcorn concept is believed to produce improved NOx
reduction. A variation of Alcorn is disclosed in PCT application
NO. PCT/GB99/00292 (published Aug. 12, 1999 as WO 99/39809)in which
upstream of Alcorn's oxidation catalyst is placed a particulate
filter and the source of reductant is positioned downstream of the
SCR catalyst and upstream of the particulate filter. The
particulate filter is disclosed as a wall-flow filter effective to
cause "combustion" at relatively low temperatures in the presence
of NO.sub.2 which is not believed especially beneficial in the
arrangement disclosed in the PCT application. U.S. Pat. No.
4,902,487 to Cooper et al. issued Feb. 20, 1990 should also be
noted for its disclosure of a particulate filter upstream of a
platinum based catalyst which arrangement is said to generate
NO.sub.2 from the exhaust gas.
SUMMARY OF THE INVENTION
Accordingly, it is one of the major undertakings of this invention
to provide an aftertreatment system configured with a Catalyzed
Soot Filter (Pt/ZrO.sub.2 --CeO.sub.2) up-stream of a zeolite (e.g.
ZNX) SCR Catalyst to produce substantially better NOx conversion
performance than the zeolite SCR Catalyst alone, especially for
higher NSR (normalized stoichiometric ratio) levels of reductant
and at lower exhaust temperatures.
Particularly, the CSF and ZNX configuration makes the SCR more
viable for LD diesel (lean burn) applications where duty cycles are
characterized by low exhaust temperatures. The CSF and ZNX SCR
configuration also exhibits better utilization of the NH.sub.3
(preferred embodiment) reductant derived from injected urea
solution than the ZNX SCR catalyst alone configuration and exhibits
zero or very low NH.sub.3 slip under all conditions. The CSF and
ZNX SCR catalyst configuration is a viable aftertreatment system
for simultaneous high level (e.g. >80%) reduction of both TPM
and NOx for diesel engines.
One aspect of this invention is to combine particulate filtration
with SCR to achieve the required high levels (>90%) of NOx and
particulate removal from diesel exhaust simultaneously and thereby
meet the objectives and overcome the emissions related problems.
Specifically, the configuration of the invention combines a
catalyzed soot filter in the exhaust up-stream of the SCR catalyst.
Although any type of CSF can be used for this invention, the
preferred type is one having a relatively high platinum (Pt)
loading. This gives good soot burning (i.e., filter regeneration)
characteristics along with other unanticipated advantages (see
synergy, below). Although either V/Ti of Zeolite SCR catalysts can
be used, a Zeolite catalyst such as ZNX is preferred because of its
excellent hydrothermal stability.
An important factor of this invention is the discovery that there
is an important synergy between the CSF and the SCR catalyst in
that the presence of the CSF up-stream of the SCR catalyst
significantly enhanced the NOx reduction performance of the SCR
catalyst. In this configuration the ZNX SCR catalyst exhibited
higher NOx conversion than the SCR catalyst alone at all
temperatures, plus it extended the effective NOx conversion range
of the ZNX SCR catalyst down to temperatures at least as low as
200.degree. C. which is well below the effective temperature range
of the ZNX catalyst alone.
It is thus an object of the invention to provide a system for
improved conversion of NOx emissions from a diesel engine, or in
another sense, for improved NOx emission conversion for any type of
internal combustion engine of a lean burn type which produces
relatively high NOx emissions.
It is another object of the invention to provide an improved
exhaust treatment system which removes particulates and reduces NOx
from diesel engine exhaust gases.
Yet another object of the invention is to provide an improved
exhaust treatment system for diesel engines which extends the lower
temperature range at which an SCR catalyst used in the system is
effective to reduce NOx emissions.
Still another specific object of the invention is to provide an
improved exhaust treatment system for diesel engines which has an
ability to better utilize an external reductant or reducing agent
in the reduction of NOx minimizing the tendency of the system to
produce reductant slip.
Still another object of the invention is to provide a simple two
part (filter & SCR) emission system sufficient to oxidize CO
and HC, reduce the particulate emissions and reduce NOx emissions
to N.sub.2. That is an oxidation catalyst (downstream of the SCR
catalyst) is strictly speaking, not necessary to meet emission
regulations. An oxidation catalyst may, however, be provided to
insure against ammonia slip which is potentially possible under
transient emission conditions. Such oxidation catalyst, if used,
would be of smaller capacity than those conventionally used to
oxidize ammonia slip occurring in conventional ammonia reductant
systems.
These and other objects, features and advantages of the invention
will become apparent to those skilled in the art upon reading and
understanding the Detailed Description of the Invention set forth
below taken in conjunction with the drawings described below.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take form in certain parts and in an arrangement
of certain parts taken together and in conjunction with the
attached drawings which form a part hereof and wherein:
FIG. 1 is a schematic arrangement depicting a 7.2 liter Heavy Duty
300 HP diesel engine exhaust system and particularly an engine
bench set-up for testing the catalyzed soot filter and urea SCR
catalyst of the present invention;
FIG. 2 is a plot of graphs of engine test results comparing NOx
conversion performance of an exhaust system using an SCR catalyst
alone and an exhaust system using CSF and SCR catalysts as a
function of NSR at 470.degree. C. inlet temperature for the engine
of FIG. 1 operated under 100% load at 1,800 rpm producing engine
out NOx of 780 ppm at space velocity of 51.33 k hr.-1;
FIG. 3 is a plot of graphs of engine test results comparing maximum
NH.sub.3 break through for an exhaust system having CSF and SCR
catalysts and an exhaust system having only an SCR catalyst as a
function of NSR under the same engine conditions set forth for FIG.
2;
FIG. 4 is a plot of graphs similar to FIG. 2 showing engine test
results comparing NOx conversion performance of exhaust systems
having only an SCR catalyst and CSF and SCR catalysts as function
of NSR at 345.degree. C. SCR inlet temperature for the engine of
FIG. 1 operated under 60% load at 1,800 rpm producing engine out
NOx of 400 ppm at space velocity of 46.94 k hr.-1;
FIG. 5 is a plot of graphs showing engine test results comparing
average and maximum NH.sub.3 break through for an exhaust system
having the SCR catalyst alone and an exhaust system having the CSF
and SCR catalysts as a function of NSR at conditions specified in
FIG. 4;
FIG. 6 is a plot of graphs similar to FIGS. 2 and 4 showing NOx
conversion performance of an exhaust system having only an SCR
catalyst and an exhaust system having the CSF and SCR catalysts as
a function of NSR at 200.degree. C. SCR inlet temperature for the
engine of FIG. 1 operated under 14% load at 1,800 ppm producing
engine out NOx of 200 ppm at space velocity of 28.33 k hr.-1;
FIG. 7 is a plot of graphs similar to that shown in FIGS. 3 and 5
of maximum NH.sub.3 breath through for an exhaust system having the
CSF and SCR catalysts and the SCR catalyst alone as a function of
NSR at engine conditions specified in FIG. 6;
FIG. 8 is a plot of graphs of test results for exhaust systems
configured with the CSF and SCR catalysts showing NOx conversion as
function of NSR (normalized stoichiometric ratio) at different
exhaust temperatures produced by the engine of FIG. 1 operated at
2200 rpm under different loads;
FIG. 9 is a plot of graphs of engine test results comparing NOx
conversion performance of emission system using the CSF and SCR
catalysts and an emission system using an SCR catalyst alone as a
function of inlet temperature for a high range of NSR values (0.61
to 0.78);
FIG. 10 is a plot of curves of ESC (13 mode OICA cycle) test
results (NOx and NSR) for an emission system using an SCR catalyst
only scaled to the normal 300 HP engine rating;
FIG. 11 is a plot of curves similar to FIG. 10 and for the ESC test
but using an emission system having a CSF and SCR catalyst
configuration scaled to the normal 300 HP engine rating;
FIG. 12 is a plot of curves similar to FIG. 10 and for the ESC test
for an emission system using only an SCR catalyst scaled to a
normal 180 HP engine rating;
FIG. 13 is a plot of curves similar to FIG. 10 and for the ESC test
for an emission system using a CSF and SCR catalyst configuration
scaled to a normal 180 HP engine rating;
FIG. 14A is a schematic depiction of the preferred embodiment of
the present invention;
FIGS. 14B and 14C are schematic depictions of possible alternative
configurations of the system of the present invention; and,
FIGS. 15 and 16 are schematic end view and section views,
respectively, of a catalyzed wall flow filter used in the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings where the showings are for the
purpose of illustrating a preferred embodiment of the invention
only and not for the purpose of limiting same, there is shown in
FIG. 1 a bench test unit 10 which does not represent the commercial
embodiment of the invention in its preferred embodiment. (The
preferred embodiment is schematically illustrated in FIG. 14A.) The
test unit 10 is shown in FIG. 1 because the unanticipated synergy
between an up-stream CSF 12 and an SCR catalyst 14 was discovered
via engine tests in an engine-dynamometer test cell depicted
schematically in FIG. 1.
The engine 15 was a Model Year 1998 Caterpillar 3126 (7.2 liter)
Direct Injected, Turbo-Charged/Intercooled engine rated at 300 HP @
2200 RPM. For the purposes of the tests the engine was calibrated
to produce 4 g/bhp-hr NOx emissions over the U.S. Heavy Duty
Transient Test Cycle.
For the tests the fuel was an ultra-low sulfur (ULS) diesel fuel
provided by Phillips Petroleum. This fuel had a nominal sulfur
content of 3 ppm.
The soot filter substrate used for the tests was an EX-80
cordierite wall-flow filter purchased from Corning Inc. The
substrate was 10.5" in diameter and 12.0" long. This filter had a
total volume of 17.03 liters (1039 in.sup.3) or 2.4 times the swept
displacement of the engine. It had a honeycomb cell spacing of 100
cpsi with a 17 mil wall thickness. The soot filter catalyst used
for the tests was the assignee, Engelhard Corporation's, filter
catalyst designated MEX 003. This catalyst is comprised of 250
g/ft.sub.3 ZrO.sub.2 applied to the soot filter substrate by
solution impregnation as zirconium acetate solution and then dried,
plus 500 g/ft.sub.3 CeO.sub.2 applied next by solution impregnation
as cerium (III) nitrate/citric acid solution (Ce:citrate mole
ratio=1:1) and then dried and calcined at 450.degree. C., plus 75
g/ft.sup.3 platinum applied by solution impregnation as
amine-solubilized Pt(II) hydroxide (i.e. Pt "A" Salt) which was
then dried and calcined at 450.degree. C. This comprised the
catalyzed soot filter in the preferred embodiment or CSF 12.
SCR catalysts 14A, 14B used for the tests were the assignee's,
(Engelhard Corporation), ZNX catalyst. Two SCR units 14A, 14B
arranged in a "Y" split are shown because FIG. 1 is a bench unit
capable of testing different catalysts so that a reference catalyst
performance can be compared to a modified catalyst. With respect to
the subject invention, both SCR catalysts 14A, 14B are identical.
The SCR catalysts 14A, 14B, each were comprised of ca. (calculated)
2 g/in.sup.3 Iron-exchanged Beta zeolite together with 4 wt %
ZrO.sub.2 binder. This catalyst was coated onto flow-thru monolith
substrates which were 10.5" in diameter and 6.0" long with a cell
spacing of 300 cpsi. Each substrate had a volume of 8.51 liters
(520 in.sup.3) for a total catalyst volume of 17.02 liters or 1040
in.sup.3.
As can be seen from FIG. 1, the exhaust from engine 15 containing
particulates and NOx is conveyed to an inlet 16 of CSF 12. On
passing through CSF 12 the particulates including soot and SOF
(soluble organic fractions) are largely removed (>90%). In
addition gas phase HC's and carbon monoxide are removed from the
exhaust by the catalyst on the soot filter. The resultant cleaned
exhaust contains primarily NOx as the main regulated emission.
Down-stream of the CSF a solution of urea in water is injected into
the exhaust, in this case via an air assisted nozzle designated
generally by reference numeral 18. The concentration of urea in the
solution was 32.5 wt % and it was delivered to the injection nozzle
via a pump. The injection rate of urea solution was regulated via
the pump rate so that the ratio of urea injected to NOx in the
exhaust could be controlled and known. As is well known, the urea
(H.sub.4 N.sub.2 CO) molecule can be decomposed by hydrolysis in
the exhaust to give ammonia (NH.sub.3) which is the active NOx
reductant. Each urea molecule yields two molecules of NH.sub.3.
Because of this 2:1 yield and for the purposes of describing the
testing and results the urea-to-NOx ratio will be referred to as
the Normalized Stoichiometric Ratio (NSR). This simply means that
for an NSR of 1 the NH.sub.3 :NOx molar ratio in the exhaust is
1:1. A 1:1 molar ratio of NH.sub.3 to NOx is the theoretical ratio
to achieve 100% NOx conversion to N.sub.2.
The exhaust stream containing the injected urea and/or ammonia
products at the desired NSR was next conveyed to the ZNX SCR
catalysts 14A, 14B. As noted above, for the tests, the exhaust flow
was split using a Y-connector 19 and conveyed to two ZNX catalysts
or bricks 14A, 14B which are mounted in parallel as shown. This
arrangement gave a total volume of SCR catalyst 14 of 17.03 liters
or 2.4 times the swept displacement of the engine. Down-stream of
the ZNX SCR catalysts 14A, 14B the exhaust streams were brought
back together via a Y-connector 20 and the exhaust gas, now cleaned
of both particulates and NOx was conveyed out of the test cell.
Sampling points for exhaust analysis are shown in FIG. 1 by lines
designated by reference numerals 22A, 22B, 22C and 22D. The normal
exhaust emission bench was used for analyzing NOx, HC's and CO. The
NOx was determined by the chemiluminescence technique. In addition,
Fourier Transform Infrared Spectroscopy (FTIR) was used to analyze
for nitrogen-species at the sampling points. The FTIR allowed for
accurate determination of NO, NO.sub.2, N.sub.2 O and NH.sub.3 in
the exhaust. Exhaust temperatures were also measured via
thermocouples at sampling points 22A, 22B, 22C and 22D.
Control tests were run for comparison with the ZNX SCR catalysts
14A, 14B alone. In this case, CSF 12 was removed from the exhaust
system and replaced by a straight pipe (not shown). A valve (not
shown) down-stream of the SCR catalysts was used to provide the
same back-pressure on the engine as when the CSF was present in
order to maintain the same engine-out NOx levels. The valve
provides an adjustable back pressure for the step load tests
discussed below.
Steady state tests were run at 1800 RPM on the engine. Engine load
was varied to achieve different exhaust temperatures. The steady
state test conditions and correspondence to drawings to be
subsequently discussed are summarized below in Table 1:
TABLE 1 Steady State Speed of 1800 rpm Exhaust SCR Cat SCR Cat
Inlet Flow GHSV NOx Load T (SCFM) (1000 Hr-1) (ppm) FIGS. 14%
200.degree. C. 285 28.3 214 6, 7 60% 345.degree. C. 471 46.9 420 4,
5 100% 468.degree. C. 515 51.3 770 2, 3
At each of these steady state conditions urea solution was injected
into the exhaust at different rates to vary the NSR level.
Emissions were measured for each NSR level and the NOx conversion
and NH.sub.3 slip (break through) determined. This was done for the
CSF and ZNX SCR catalyst configuration and the ZNX SCR Catalyst
alone configuration. The results are discussed below. The results
based on the FTIR measurements are shown, but these were in good
agreement with the chemiluminescence results.
FIG. 2 shows the NOx conversion levels as a function of NSR for the
CSF and ZNX SCR configuration indicated by the trace passing
through circles designated by reference numeral 30 and for the ZNX
SCR catalyst configuration alone indicated by the trace passing
through diamonds designated by reference numeral 31 at the 100%
load/468.degree. C. SCR inlet condition. As can be seen there
appears to be a slight advantage for the CSF and ZNX catalysts
configuration, but the NOx conversion performance of both systems
is very similar. The NOx conversion levels are essentially at or
slightly above theoretical for the calculated NSR level thus
showing very high level utilization of the urea reductant and
thereby very high NOx conversion. Note from Table 1 that the
exhaust inlet temperature of 468.degree. C. is well within the ZNX
SCR catalyst temperature window for optimum catalyst activity. The
addition of CSF catalyst 12 does not materially change the NO.sub.2
conversion efficiency which would be expected. That is, one would
expect the SCR catalyst to perform within its operating temperature
window and improved results by addition of an upstream catalyst
should not occur.
FIG. 3 shows the maximum NH.sub.3 break through levels as a
function of NSR for the same runs at 100% load/468.degree. C. SCR
inlet condition. As can be seen NH.sub.3 break through for the ZNX
SCR catalyst alone configuration is very low to at least an NSR
level of ca. 0.7. However, at an NSR of ca. 0.96 the ZNX alone
configuration indicated by the trace passing through diamonds
designated by reference numeral 34 exhibits a maximum NH.sub.3
break through of nearly 40 ppm. The goal should be to keep NH.sub.3
slip at all times below ca 20 ppm and preferably below 10 ppm. The
CSF and ZNX configuration, on the other hand, indicated by the
trace passing through diamonds designated by reference numeral 35
exhibited no (0 ppm) NH.sub.3 break through at all NSR levels. This
is somewhat surprising because both systems (SCR along and CSF and
SCR showed similar NOx conversion ranges) and shows a clear
advantage to the continuation of CSF catalyst 12 upstream of SCR
catalyst 14 for preventing ammonia slip.
FIG. 4 shows NOx conversion as a function of NSR for the CSF 12 and
SCR 14 catalyst arrangement which is shown as the trace passing
through circles designated by reference numeral 36. When the
emission system was only SCR 14 by itself, the NOx conversion is
shown as the trace passing through diamonds designated by reference
numeral 37. Traces 36, 37 were developed with engine 15 at the 60%
load/345.degree. C. SCR inlet condition. As can be seen, at low NSR
ratios, CSF and ZNX SCR trace 36 exhibits only a slight advantage
in NOx conversion over ZNX alone trace 37. However, as the NSR
ratio is increased to obtain higher NOx conversion, the performance
advantage of the CSF and ZNX SCR configuration also increases. At
the highest NSR levels evaluated (>0.9) the NOx conversion of
the ZNX SCR alone configuration appears to be leveling off at ca.
60%. It should be noted that the temperature for the steady state
condition plotted in FIG. 4, lies at the lower edge of the
temperature window for optimum conversion activity for the ZNX
catalyst alone. The NOx conversion for the CSF and ZNX SCR catalyst
configuration, as shown in FIG. 4, is nearly 100%. This shows a
clear improvement in the SCR reaction by the presence of CSF
catalyst 12 up-stream of the ZNX SCR catalyst 14. It also shows
excellent utilization of the NH.sub.3 derived from urea for the CSF
and ZNX configuration.
In FIG. 5 which was generated with the engine at 60%
load/345.degree. C. SCR inlet temperature condition, the maximum
NH.sub.3 break through is indicated by a trace passing through
diamonds designated by reference numeral 40 and the average.
NH.sub.3 break through is indicated by a trace also passing through
diamonds but designated by reeference numeral 41 NH.sub.3 break
through for the ZNX SCR catalyst alone configuration. Also plotted
is the maximum NH.sub.3 break through for the ZNX SCR catalyst
configuration as shown by the trace passing through circles
designated by reference numeral 42. At this temperature and
condition the ZNX SCR catalyst alone configuration exhibits much
increased NH.sub.3 break through, especially at NSR levels greater
than ca. 0.55. This is consistent with what would be expected from
studying FIG. 4 which shows a leveling off of the NO.sub.x
conversion performance at higher NSR ratios as a result of poorer
utilization of NH.sub.3 from urea. In marked contrast, the maximum
NH.sub.3 break through for the CSF and SCR configuration was zero
at each of the NSR levels tested as shown by trace 41. This is also
consistent with FIG. 4 which shows for higher NSR ratios a high
NO.sub.x conversion and thus full utilization of NH.sub.3 from
urea.
FIG. 6 shows, with engine 15 at the 14% load/200.degree. C. SCR
inlet condition, the NOx conversion as a function of NSR for the
CSF and SCR catalysts as a trace passing through circles designated
by reference numeral 44 and the SCR catalyst alone configuration as
a trace passing through diamonds designated by reference numeral
45. At this condition the Nox conversion with the ZNX SCR catalyst
alone configuration is rather low (10-15%) and is essentially
unresponsive to changes in NSR level. The exhaust temperature
(200.degree. C.) is well below the temperature window normally
observed for the ZNX SCR catalyst activity. However, with CSF 12
up-stream of the ZNX SCR catalysts 14 good NOx conversion was
observed. The NOx conversion increased with increasing level of NSR
until it leveled off at ca. 70% for NSR above ca. 0.63.
Specifically, FIG. 6 shows that SCR catalyst 14 is catalytically
active at light engine loads producing low temperatures of 200EC in
that at least 50% of NOx emissions are reduced to N2 by SCR
catalyst. As clearly shown by trace 45, ZNX SCR catalyst 14 is not
normally catalytically active at this temperature at the space
velocities measured. It is now possible to use the CSF and SCR
catalyst configuration for low load diesel driving conditions, such
as are encountered for LD diesel autos or SUV's.
FIG. 7 shows, at the 14% load/200.degree. C. SCR inlet condition,
the maximum NH.sub.3 break through as a function of NSR for the CSF
and ZNX SCR catalyst configuration indicated by a trace passing
through circles designated by reference numeral 48 and the ZNX SCR
catalyst alone configuration indicated by a trace passing through
diamonds designated by reference numeral 49. As can be seen, the
ZNX SCR alone configuration, trace 49, exhibits NH.sub.3 break
through above an NSR level of ca. 0.62 and the NH.sub.3 break
through becomes very high at NSR levels above 0.9. The CSF and ZNX
configuration of the present invention, trace 48, exhibited zero
NH.sub.3 break through for all levels of NSR which were
evaluated.
FIG. 8 is a summary graph and shows NOx conversion as a function of
NSR for the CSF and ZNX SCR catalyst configuration of the present
invention at all three of the steady state conditions (100%, 60%
& 14% load) discussed above. These are the same results shown
in FIGS. 2, 4 & 6, but plotted on the same chart and the traces
carry the same reference numerals previously described. As can be
seen the NOx conversion as a function of NSR is very similar at
each of the test conditions--exhaust temperatures of 470, 345 &
200.degree. C. Furthermore, the NOx conversion levels are at or
above the theoretical for calculated NSR with the exception of ca.
0.86 NSR at the 200.degree. C. SCR catalyst inlet temperature shown
as trace 44.
FIG. 9 shows a view of the results from a different perspective.
This figure plots NOx conversion as a function of SCR catalyst
inlet temperature for some of the higher NSR ratios between
0.61-0.78. For these higher ratios, the performance of the
inventive configuration of downstream CSF 12 and upstream SCR 14 is
shown by a trace passing through circles designated as reference
numeral 50 while the performance of an emission system equipped
only with a SCR catalyst 14 is shown by a trace passing through
diamonds designated by reference numeral 51. The traces 50, 51 show
the clear performance advantage of the CSF and SCR configuration
over the SCR alone configuration at lower exhaust inlet
temperatures. Similar but not as dramatic curves can be plotted at
lower values of NSR. Because lower NSR ratios are not likely to be
used in commercialization of the invention, they are not shown.
That is the invention (which typically does not use an NSR ratio
higher than 1) utilizes NSR ratios in the ranges depicted, i.e.,
between 0.61 to 1.0 so that the reductant is efficiently utilized
and sizing of SCR is minimized. However, some improvement will
occur at lower ranges and improvement at higher NSR ratios
approaching 1.5 is expected.
Engine tests were also run using the more dynamic Euro III test
cycle which is also referred to as the OICA Cycle or European
Stationary Cycle (ESC). This test cycle is comprised of 25, 50, 75
& 100% loads for three different speeds (12 test conditions
total) under the engine's torque curve, plus idle (1 test
condition). Examples of the key results obtained from the ESC tests
are discussed, below.
FIG. 10 shows the OICA Cycle Results for the ZNX SCR catalyst 14
alone configuration with the conditions scaled to the normal 300HP
engine rating. For this test the average SCR inlet exhaust
temperature was 357.degree. C. The chart shows the mode-by-mode NOx
conversion and RSR level used. More particularly, the NOx
conversion, read from the left y-axis, is shown by a trace passing
through circles designated by reference numeral 60 and the NSR,
read from the right y-axis, is shown by a trace passing through
diamonds designated by reference numeral 61. For an average NSR
level of ca. 0.985 the weighted NOx conversion over the test cycle
was 67.3%. The maximum NH.sub.3 slip observed over the cycle was
241 ppm.
FIG. 11 shows for comparison the results for the same OICA Cycle as
in FIG. 10, but for CSF 12 and ZNX SCR catalyst 14 configuration.
In FIG. 11, the NOx conversion, read from the left y-axis, is shown
by a trace passing through circles designated by reference numeral
64 and the NSR value, read from the right hand y-axis, is shown by
a trace passing through diamonds designated by reference numeral
65. As can be seen for a comparable average SCR inlet temperature
of 367.degree. C. and an average NSR of 0.976 the average weighted
cycle NOx conversion observed was 85.1% --nearly 20% higher than
for the ZNX SCR alone configuration. There was no NH.sub.3 break
through observed over this test cycle.
The OICA Cycle tests were repeated, but with scaling of the load
points to a 180 HP engine rating. In effect this reduced the
average exhaust temperatures and lowered total exhaust flows.
FIG. 12 shows the OICA Cycle results for the ZNX SCR catalyst 14
alone configuration. The NSR trace passes through diamonds
designated by reference numeral 66 and the NOx trace passes through
circles designated by reference numeral 67. As can be seen for an
average SCR catalyst inlet temperature of 288.degree. C. and an
average NSR level of 0.921 a weighted average NOx conversion over
the test cycle of 58.2% was obtained. This was ca. 9% lower than
for the same configuration for the 300 HP test with average
temperature of 357.degree. C. The maximum NH.sub.3 slip observed
over the test cycle was 310 ppm.
FIG. 13 shows the OICA Cycle results for the CSF 12 and SCR 14
catalysts configuration at the 180 HP rating. The NSR trace passes
through diamonds designated by reference numeral 68 and the NSR
trace passes through squares designated by reference numeral 69. As
can be seen for an average SCR catalyst inlet temperature of
296.degree. C. and with an average NSR of 0.963 a weighted average
NOx conversion of 89.9% over the cycle was obtained. This was
slightly better than for this configuration at the 300HP rating at
an average 367.degree. C. which temperature, in turn, is better
matched to the ZNX SCR catalyst activity window. However, its
possible that the lower exhaust flows and thereby lower GHSV's
(space velocity through the SCR) for the 180 HP condition
compensated for the lower temperature condition. For the 180 HP
condition the CSF and ZNX SCR configuration gave over 30% higher
weighted average cycle NOx conversion than did the ZNX SCR alone
configuration. The CSF and ZNX configuration exhibited no NH.sub.3
break through over the test cycle.
The OICA Cycle results also showed that final HC emissions were
reduced significantly by either configuration. However, with the
CSF up-stream the HC's were removed prior to the SCR catalyst but
with the ZNX SCR catalyst alone configuration both HC's and NOx had
to be converted over the SCR catalyst. The ZNX SCR catalyst alone
configuration exhibited little CO conversion as might be expected.
With CSF catalyst 12 up-stream, a high level of CO conversion was
obtained over CSF catalyst 12 presenting a low CO exhaust to SCR
catalyst 14. This is probably not that significant for SCR catalyst
activity, but overall, tailpipe CO is substantially decreased with
the CSF present.
Table 2 set forth below summarizes the ESC test as follows:
TABLE 2 ESC Test Data Avg. Max Temperatures, deg C. NOx NH.sub.3
SCR Out Emissions, Config- CSF SCR SCR Avg. Conv. Slip g/kW-hr
uration In In Out NSR % ppm HC CO NOx FIG. 10 357 368 0.985 67.3%
241 0.04 0.83 2.07 SCR 300 HP FIG. 11 399 367 376 0.976 85.1% 0
0.03 0.07 0.92 CSF + SCR 300 HP FIG. 12 288 300 0.921 58.2% 310
0.07 1.09 2.51 SCR 180 HP FIG. 13 321 296 303 0.963 89.9% 0 0.04
0.1 0.61 CSF + SCR 180 HP
The results of the OICA Cycle tests were consistent with the steady
state tests showing improved performance of the CSF and SCR
catalysts configuration compared with the SCR catalyst alone
configuration for total NOx conversion and control of NH.sub.3
break through.
1) In general summary, the Steady State Tests showed:
a) A slight advantage of CSF and SCR over SCR alone as a function
of NSR at high inlet temperature (470.degree. C.). Both
configurations attained ca.80-90% NOx conversion for
NSR=0.8-0.9;
b) For lower inlet temperatures (345.degree. C. & 200.degree.
C.) the CSF and SCR configuration gave substantially better NOx
conversion than the SCR alone configuration at all NSR levels, but
especially at higher NSR's. CSF and SCR attained 70-90% NOx
conversion for NSF=0.7-0.9. Activity for NOx conversion for SCR
alone decreased with decreasing inlet temperature while CSF and SCR
maintained activity;
c) The CSF and SCR system gave 70% NOx conversion at 200EC for NSR
0.7-0.85 and SCR alone configuration gave only 10%. CSF and SCR is
therefor viable for light load and light duty diesel
applications;
d) The more effective utilization of the reductant by this
invention results in less unreacted ammonia leaving the catalyst.
In these experiments virtually all of the available reductant
(ammonia) was used to reduce NOx and therefore no unreacted ammonia
could be detected at the catalyst exit; and,
2) The ESC Cycle Testing showed:
e) The 300 HP rated modes resulted in an average exhaust
temperature of ca. 360.degree. C. The SCR catalyst alone (two
parallel bricks) gave 67% weighted cycle NOx reduction for NSR 0.98
with NH.sub.3 slip. The CSF and SCR (two parallel bricks) system
gave 85% weighted cycle NOx reduction for NSR's of 0.98 with no
NH.sub.3 slip; and,
f) The 180 HP rated modes resulted in an average exhaust
temperature of 290.degree. C. Weighted cycle NOx reduction of 58%
was attained with SCR alone at NSR=0.92 with NH.sub.3 slip. The CSF
and SCR (two bricks in parallel) system gave 90% weighted cycle NOx
reduction for NSR=0.96 with no NH.sub.3 slip.
In general summary, the tests discussed above showed a clear
performance advantage for the CSF and SCR catalysts configuration
compared with the SCR catalyst alone configuration, especially with
respect to NOx conversion, NH.sub.3 utilization and NH.sub.3 break
through at low exhaust temperatures and at higher NSR levels. While
the inventors do not intend necessarily to be bound by any
particular theory, there are several reactions which may contribute
to the unexpected results disclosed above.
First CSF catalyst 12 is removing the particulates (carbon soot and
liquid HC SOF's (soluble organic fractions)) from the exhaust
before it can reach the SCR catalyst. It's possible that this
particulate material could deposit on the SCR catalyst 14 and
reduce its effectiveness via fouling or occupation of active
catalyst sites. The removal of the particulates could thus be an
advantage. In addition the CSF gives a high conversion of gas phase
hydrocarbons before they encounter the SCR catalyst. These HC's
could also occupy catalyst active sites thereby interfering with
the SCR activity.
CSF 12 used for the tests was formulated with a relatively high Pt
loading level (75 g/ft.sup.3). FTIR exhaust emissions analysis
showed that consistent with known operating characteristics of
diesel engines, the engine-out NOx was primarily in the form of NO
with a very small level of NO.sub.2. Thus, the NO.sub.2 /NOx ratio
was very low. This was the nature of the NOx entering CSF catalyst
12. The exhaust gas coming out of CSF catalyst 12, however, showed
significantly higher levels of NO.sub.2 and the NO.sub.2 /NOx ratio
was also higher than engine-out. That is, the nature or composition
of the NOx entering ZNX SCR catalyst 14, i.e., at 22B, had a higher
concentration of NO.sub.2 than that emitted from engine 15, i.e.,
at 22A. The NO.sub.2 molecule is generally considered to be a more
reactive species than the NO molecule. Further, NO.sub.2 is more
polar and thus potentially more adsorbable on catalyst surfaces
than NO. Thus, exhaust gases having a NOx composition with a higher
NO.sub.2 /NOx ratio may exhibit enhanced NOx reduction activity in
the SCR reaction. The NO.sub.2 /NOx ratios for the steady state
test conditions described above at the various sampling points
(shown in FIG. 1) are given in Table 3 below:
TABLE 3 NO.sub.2 /NOx Ratios Engine- Engine- Out CSF-Out ZNX-Out
(NSR Out NO.sub.2 /NOx NO.sub.2 /NOx >0.8) NOx Ratio Ratio
NO.sub.2 /NOx Ratio Load Temp (ppm) 22A 22B 22C, 22D 100%
468.degree. C. 770 0.3% 12.7% 0.0% 60% 345.degree. C. 420 1.2%
45.4% 0.0% 14% 200.degree. C. 214 4.6% 28.2% 0.0%
The enhanced levels of CSF-out NO.sub.2 can be seen in Table 3 as a
significant increases in the NO.sub.2 /NOx ratio for each of the
steady state test conditions. Furthermore, no NO.sub.2 could be
found in the SCR catalyst-out sampling position. Thus, 100% of the
NO.sub.2 was converted over SCR catalyst 14.
The preferred embodiment uses a solution of urea in water injected
into the exhaust. FIG. 1 is schematically reproduced in FIG. 14A in
its commercially implemented sense and reference numerals used in
FIG. 1 will apply to FIG. 14A where possible. As is well known,
various arrangement are used in which aqueous urea shown on one
line 70 with air on another line 71 are mixed in various nozzle
configurations shown as mixing station 72 to pulse or meter
(schematically indicated by valve 74) a precise amount (stated as
an NSR value) of ammonia which is injected as a spray from a nozzle
into the exhaust stream. Valve 74, in turn, is controlled or
regulated by a computer (not shown) typically the engine's ECM
(electronic command module) which interpolates sensor (not shown)
readings of the exhaust gases to establish a reductant flow
sufficient to match a desired NSR value. It is known that an
aqueous urea solution tends to lower the exhaust temperature which
is not desirable because of the SCR active window temperature
range. The data has shown, however, that the temperature at which
the SCR is catalytically active is lowered (at space velocities
indicated) if the inventive arrangement is used. Thus, the
invention can function with an aqueous urea solution in the
preferred embodiment because the adverse effects of dropping
exhaust gas temperature is not as harmful to the reduction system
as it would otherwise be, i.e., an arrangement without CSF 12.
However, the invention is not limited to urea mixed with water and
contemplates use of a solid ammonia reductant because it is (among
other reasons) not desirable to lower exhaust gas temperature even
with the invention. Accordingly, urea prills could be injected or
supplied on one line 70 with heat (optionally by means of a carrier
gas, i.e., exhaust gas) on another line 71 to the mixing station
72. The ammonia in gaseous form is injected by pulse metering
through a valve such as valve 74 to the exhaust stream in FIG. 14A.
As is well known, a gasified solid reductant does not reduce
exhaust gas temperature. Also, any ammonia precursor can be used in
the preferred embodiment.
The invention has been demonstrated to work with nitrogen
containing reductants and it is noted that an SCR catalyst is
generally a term associated with nitrogen reductants. The inventors
believe that the invention may have application to reductants other
than nitrogen reductants although they have not tested the
invention as of the date hereof to verify their belief. In any
event, the term "SCR" catalyst will be used herein in a broader
sense to mean a selective catalytic reduction in which a catalyzed
reaction of nitrogen oxides with a reductant occurs to reduce the
nitrogen oxides. "Reductant" or "reducing agent" is also broadly
used herein to mean any chemical or compound tending to reduce NOx
at elevated temperature. In the preferred embodiment, the reducing
agent is ammonia, specifically an ammonia precursor, i.e., urea and
the SCR is a nitrogen reductant SCR. However, in accordance with a
broader scope of the invention, the reductant could include fuel,
particularly diesel fuel and fractions thereof as well any
hydrocarbon and oxygenated hydrocarbons collectively referred to as
an HC reductant. Therefore, in FIG. 14A, fuel oil on one line 70,
could be supplied and air, optionally, on the other line 71, and
the fuel/air mixture cracked in mixing station 72 (to produce the
reductant) and pulsed metered through valve 74 to the SCR (as
broadly defined). Alternatively, the reductant (fuel oil) can be
metered in liquid form, i.e., sprayed, into the exhaust gas.
Definition notwithstanding, when a hydrocarbon reductant is used to
reduce NOx over a catalyst, the catalyst is typically referred to
as a lean NOx catalyst and lean NOx catalysts are typically
classified as either a low temperature NOx catalyst or a high
temperature NOx catalyst. The low temperature lean NOx catalyst is
platinum based (Pt-based) and does not have to have a zeolite
present to be active, but Pt/zeolite catalysts are better and
appear to have better selectivity against formation of N.sub.2 O as
a by-product than other catalysts, such as Pt/alumina catalysts.
Generally a low temperature lean NOx catalyst has catalytically
active temperature ranges of about 180 to 350.degree. C. with
highest efficiencies at a temperature of about 250.degree. C. High
temperature lean NOx catalysts have base metal/zeolite
compositions, for example Cu/ZSM-5. High temperature NOx catalysts
have a lower temperature range of about 300-350.degree. C. with
highest efficiency occurring around 400EC. The broader scope of
this invention uses either high or low temperature lean NOx
catalysts with an HC reductant, as described for example in FIG.
14A. Because of the potential for ammonia to form NOx, it is
considered desirable to introduce the ammonia to the exhaust gas at
the in-between position shown in FIG. 14A. However, an HC reductant
does not raise the same concerns so that an HC reductant can be
introduced into the exhaust gas as shown in FIG. 14B. Further, it
is therefore possible to construct a single catalyst brick 13 which
has a catalyzed soot filter at its entrance portion and a lean NOx
catalyst extending over its exit portion as shown in FIG. 14C with
the HC reductant introduced to the exhaust gases at the inlet of
the combined catalyst. The catalysts could, of course, be separate
and combined in a single cover. Again, as of the date of this
invention, specific tests using lean NOx catalysts have not been
performed. However, based on observations during testing of the
preferred embodiment using ammonia reductant, it is believed that
comparable results may be obtained using lean NOx catalysts with an
HC reductant.
The reason why the CSF and nitrogen reductant SCR embodiment is
preferred can be demonstrated by reference to FIGS. 15 and 16 which
schematically show end and side views, respectively, of a wall flow
filter 80. The porous or gas permeable walls of wall flow filter 80
form channels with the interior surface of any given wall forming a
portion of a channel and the exterior surface of the same wall
forming a portion of an adjacent channel. Channels in the wall flow
filter have the conventional checkerboard pattern (FIG. 15) which
have alternating closed 81 and open 82 channels to the entry side
of the exhaust (FIG. 16). All channels are catalyzed as discussed
above for purposes of explanation. (Note that it may be possible to
selectively coat portions of channels 81, 82. Again, most of the
NOx in the exhaust gas produced by diesel engine 15 is NO as
discussed above. While the composition of the exhaust gases can be
varied by any number of factors such as by fuel choice, fueling,
combustion chamber design, etc., typically NO will comprise at
least 50% of the NOx discharged from the engine's combustion
chamber.) Nitric oxide, NO, and soot enter open channels 82. It is
believed NO oxidizes by reaction with the catalyzed surface on
inlet channel 82 and changes to NO.sub.2. As is well known, soot
gets trapped by wall flow filter walls which allow exhaust gas to
pass therethrough as shown by arrows 90. However, NO.sub.2 formed
in inlet channel 82 reacts with the soot trapped on each inlet
channel's walls and reduces to NO. Reaction by NO.sub.2 with soot
is beneficial to the filter (maintains cleaner filter, less
backpressure, etc.) and to the emission process. (NO.sub.2 is
highly reactive with carbonaceous material.) Nitric oxide, NO
entering closed channel 81 now reacts with catalyst on the wall
surfaces of closed channel 81 and oxidizes to NO.sub.2. The
NO.sub.2 produces benefits allowing enhanced operation of the
nitrogen reductant SCR as described above. This is a distinction
over the prior art arrangements discussed above which used a DOC
(diesel oxidation catalyst), either by itself upstream of the SCR
or upstream of a particulate filter and the SCR. In these
arrangements, the DOC is exposed to and possibly subject to
clogging from soot. It is of little benefit because NO.sub.2
produced in the DOC reduces to NO upon contact with soot in the
particulate filter. Also, a DOC upstream of the SCR and downstream
of a particulate filter has little benefit if the catalyzed
particulate filter is properly sized. The costs of such emission
arrangement is needlessly increased because of the requirement of
the DOC.
The composition of CSF 12 in the preferred embodiment has been
described above. As schematically indicated in FIGS. 15 and 16, the
catalytic material is deposited on a carrier of a type usually
referred to as honeycombed or a monolith carriers comprising a
unitary body, generally cylindrical in configuration, having a
plurality of fine, substantially parallel gas flow passages or
channels extending therethrough. When the channels are open-ended,
the carrier is referred to as a "flow through" carrier. When each
channel is blocked at one end of the carrier body, with alternate
channels blocked at opposite end-faces the carrier is referred to
as a wall-flow carrier (or filter). The wall-flow carrier as well
as the catalytic material deposited thereon is porous so that
exhaust gases can flow through the walls of the carrier (and
without creating excessive backpressure on the engine). The
monolithic carrier body is preferably comprised of ceramic-like
materials such as cordierite, %-alumina, silicon nitride, zirconia,
mullite, spodumene, alumina-silica-magnesia or zirconium silicate.
The catalyst coated or dipped or sprayed onto the carrier, (other
than the composition) specifically mentioned above may be of a
composition such as disclosed in assignee's U.S. Pat. No. 5,100,632
to Dettling et al., issued Mar. 31, 1992, entitled "Catalyzed
Diesel Exhaust Particulate Filter" or even the catalyst composition
utilizing zeolites disclosed in assignee's U.S. Pat. No. 5,804,155
to Farrauto et al., issued Sep. 8, 1998, entitled "Basic Zeolites
as Hydrocarbon Traps for Diesel Oxidation Catalysts". Both the '632
and '155 patents are incorporated by reference herein for their
disclosure of the catalyst compositions applied to the carrier of
the CSF used in this invention. As noted above, the diesel exhaust
is a heterogeneous material which contains pollutants such as
carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides
(NOx) as well as soot particles. Soot particles compose both a dry,
solid carbonous fraction and a soluble organic fraction. The
soluble organic fraction is sometimes referred to as a volatile
organic fraction (VOF or SOF) which may exist in diesel exhaust
either as a vapor or as aerosol (fine droplets of liquid
condensate) depending on the temperature of the exhaust gas. The
catalyst on the CSF oxidizes the VOF retarding or minimizing CSF
blockage or inhibiting decrease in permeability of the wall-flow
filter's channels. The soot filter also oxidizes HC and CO to
convert these pollutants into "benign" emissions. The gases
produced from the oxidation of VOF are generally non-polluting and
do not materially interfere with or block the active sites of the
SCR catalyst. As noted, the CSF catalyst also oxidizes nitric
oxide, NO, to NO.sub.2 which on contact with VOF readily reduces to
NO and is thus beneficial to the life of the CSF catalyst. Once the
NO passes through the channel wall it again contacts the catalyst
and oxidizes to the NO.sub.2 state which is believed beneficial to
the SCR catalyst reduction process for reasons noted.
In the preferred embodiment discussed above, a high loading of the
precious metal coating (platinum group metal which is mixed with an
alkaline earth metal oxide such as magnesium oxide) was used in the
experiments, i.e., 75 g/ft.sup.3. As discussed in the Background,
the invention has application to diesel engines and diesel engines
operate at lean fueling conditions. As a matter of definition, lean
fueling condition means there is sufficient oxygen mixed with fuel
to produce at least stoichiometric combustion of the fuel. Because
excess oxygen is usually present, generally the HC and CO emissions
from a diesel engine are less than those produced by a gasoline
powered engine which typically cycles between rich and lean
conditions and uses a TWC catalyst (three way catalyst). While the
HC and CO emissions may be reduced in quantity in a diesel engine,
considering that the invention uses the CSF as the primary source
of converting HC and CO emissions to "benign" emissions and
increases the NO.sub.2 to a level having a noticeable affect on the
ability of the SCR to reduce NOx, a high loading of precious metal
coating on the CSF is desired, preferably in the range of at least
50 g/ft.sup.3 and not less than about 25 g/ft.sup.3. In
applications having a DOC downstream of the SCR sized to convert
reductant slip (ammonia slip), improved performance of the SCR is
expected to occur with lesser concentrations of precious metal
coating. In fact, improved performance of the SCR could occur if
the precious metal, i.e., platinum concentrations are as low as 5
g/ft.sup.3. NOx reduction will improve as the concentration of
platinum increases on the catalyst substrate. The optimum precious
metal concentration, however, is a function of a number of factors
including the fuel composition, the engine design, engine
operation, emission regulations, etc.
The invention has been described with reference to the assignee's
ZNX SCR catalyst which has enjoyed commercial success for NOx
reduction at gas temperatures occurring within its temperature
window whereat the SCR catalyst is catalytically active for space
velocities (flow rate of exhaust gas through the SCR catalyst)
normally produced by mobile diesel engines. Other nitrogen
reductant SCR catalysts compositions such as are disclosed in
assignee's U.S. Pat. No. 4,961,917 to Byrne, issued Oct. 9, 1990,
entitled "Method for Reduction of Nitrogen Oxides with Ammonia
using Promoted Zeolite Catalysts" or the staged catalyst
composition disclosed in assignee's U.S. Pat. No. 5,516,497 to
Speronello et al., issued May 14, 1996, entitled "Staged
Metal-Promoted Zeolite Catalysts and Method for Catalytic Reduction
of Nitrogen Oxides Using the Same", may be employed. The '917 and
'497 patents are incorporated herein by reference for their
disclosure of SCR compositions. Generally, the references show a
catalyst composition of zeolite, a promoter selected from the group
consisting of iron and copper and a refractory binder. This is the
preferred composition of the SCR catalyst and the ZNX SCR catalyst
composition disclosed above falls within this general
classification. However, a Vanadium-Titantium catalyst may also be
acceptable and reference for a typical composition of such catalyst
may be found in U.S. Pat. No. 4,833,113, issued May 23, 1989 to
Imanari et al., entitled "Denitration Catalyst for Reducing
Nitrogen Oxides in Exhaust Gas", also incorporated by reference
herein.
As indicated above, light diesel engines have lower exhaust gas
operating temperature ranges than heavy duty diesel engines. As a
matter of distinction or characterization and generally speaking,
the lower normal operating temperature range of light duty diesel
engines (i.e., diesel engines on autos, SUVs, pick-up trucks)
produce exhaust gases in the temperature range of 150-250.degree.
C. in contrast to the lower normal operating temperature range of
heavy duty diesel engines in vehicles such as trucks which may be
in the range of 235-500.degree. C. Peak temperatures are
considerably higher. As shown and for the same space velocity, the
ZNX SCR catalyst with the upstream CSF catalyst becomes
catalytically active at lower temperatures than those temperatures
at which the ZNX SCR catalyst would become catalytically active if
directly exposed to the combustion gases produced in the combustion
chambers of engine 15 (i.e., the exhaust gases). Any SCR catalyst
using a nitrogen reductant will have a lower catalytically active
temperature (at the same space velocity) when used in the
arrangement of the invention. Further, the tests show that the
reduction in the catalytically active temperature of the ZNX
catalyst was not accompanied with any noticeable reduction in the
efficiency of the ZNX SCR catalyst. Thus, the invention has
specific application to light duty diesel engine applications.
In a specific embodiment of the present invention there is a
wall-flow type catalyzed soot filter adjacent to the diesel engine.
A valve is downstream of said soot filter's exit in fluid
communication with a nitrogen reductant and with said exhaust gases
after exiting said soot filter. There is a means for regulating
said valve to control the quantity of said nitrogen reductant
admitted to said exhaust gases. A nitrogen reductant SCR catalyst
is downstream of the valve and said soot filter. The SCR catalyst
has a set temperature at which said SCR catalyst becomes
catalytically active for a set space velocity if said exhaust gases
pass through said SCR catalyst with a set quantity of reductant
immediately upon exit from said engine that is higher than the
temperature at which said SCR catalyst becomes catalytically active
when said exhaust gases pass through said SCR catalyst at said set
space velocity with said set quantity of reductant after passing
through said soot filter.
The invention has been described with reference to a preferred
embodiment. Obviously, modifications and alterations will occur to
others upon reading and understanding the detailed description of
the invention. It is intended to include all such modifications
insofar as they come within the scope of the present invention.
* * * * *